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I disagree. How could you claim the flag if it is possible for your opponent to tie you?Because in case of a tie the flag will go to the player who did not play the last card.
I don't have the rules in front of me to double check, but I agree with Brodie that a flag can be claimed if it can be shown that your opponent cannot beat your formation (tactics cards are ignored for this purpose)
Note that this includes the case where a flag can be claimed without your opponent playing a single card; All you need is a colour matched 8-9-10.
Of course your opponent should at least dump a card there as soon as they see that coming.